Acoustic imaging is used as an adjunct to optical imaging in media where light is either unavailable or severely attenuated. Underwater acoustic imaging is generally used in place of optics at ranges greater than 10 meters, when water conditions are turbid, or when ambient light is insufficient and it is impractical to provide artificial light. Medical ultrasound is used in place of optics for inexpensive noninvasive imaging of and through fluid bathed tissue and organs.
In both underwater acoustic imaging and medical ultrasound, acoustic images are constructed by transmitting acoustic energy into an environment (typically but not necessarily fluid filled) and then measuring and sorting the received echoes according to their spatial origin. This sorting is usually accomplished with either an acoustic lens or a beam former through a process known as spatial filtering. Acoustic imaging is similar in process to optical imaging, however acoustic wavelengths at frequencies practical for ocean or medical imaging are considerably larger than optical wavelengths and therefore acoustic image resolution is comparatively quite poor, even with much larger physical apertures.
Most attempts to improve the spatial resolution of acoustic imaging systems have focussed on increasing the size of the aperture in the system. While it is possible to construct acoustic imaging systems which have large apertures and therefore have improved spatial resolution, such systems are physically large and complicated. They would not be appropriate, for example, for use on a small autonomous underwater vehicle.
Prior art methods for acoustic imaging may be divided into two main categories, beam former or lens based imaging and spatial modelling based imaging. Beamformer or lens based imaging methods presume a full and continuous angular arrival spectrum. The angle of arrival of a reflected signal is measured directly by performing spatial filtering to sort the entire spectrum into bins. The angular extent of each bin (beamwidth) decreases approximately as the inverse of the number of equi-spaced array elements. In the simplest example of beamformer imaging, a single point scatterer is excited monostatically by a short, narrow band acoustic pulse. The resulting backscatter is received by an N element linear array of transducers with half wavelength element spacing ##EQU1## Scanning a beam, formed from the array elements, across the scatterer will produce a two dimensional image with range resolution determined by the acoustic pulse length and angular resolution governed by the array aperture size (e.g. by N).
A spatial modelling approach, on the other hand, is able to utilize as few as 2 array elements to produce a similar two dimensional image by estimating the backscatter wave number (i.e. angle-of-arrival (AOA)). The range resolution remains the same as for the beamformed case however, the angular resolution is no longer restricted by the aperture size and instead is limited only by the adherence of the signal to the underlying model assumptions and the display pixel size. The disadvantage of spatial based modelling is that the image will be valid only when the underlying model is satisfied. Artifacts of various kinds may be created if the underlying model is not satisfied.
An example of a successful spatial modelling based imaging method is sidescan sonar. In sidescan sonar a short acoustic is pulse is transmitted from a single transducer array. The array is designed to generate a fan-shaped beam that is quite narrow in the horizontal direction. The beam is oriented in a side-looking geometry. After an acoustic pulse has been transmitted the array detects a backscattered signal from the narrow strip of seafloor illuminated by the transducer beam pattern. The signal reflected from the seafloor is detected by the transducer. In sidescan sonar the angle of arrival of the backscattered signal is not directly measured. Instead, the acoustic angle-of-arrival, .theta., is inferred from the time of arrival of the reflected signal. That is, the angle of arrival is assumed to be a function of range, R. The function can be derived from the known sonar altitude, H and the assumption that the seafloor is predominantly flat. Successive strips are imaged by plotting the backscatter intensity in x at constant y and then advancing the sensor platform along the Y axis and repeating the transmit/receive operation. The resulting two dimensional image can be quite spectacular with an angular image resolution that is far superior to that achievable via beamforming. Sidescan sonar is widely used for qualitative seafloor exploration and search applications. A problem with sidescan imaging is that the sea floor is, in general, not flat. This departure from the model can result in the generation of various artifacts in a sidescan sonar image and makes sidescan sonar unsuitable for making quantitative measurements.
A variation of sidescan imaging is swath bathymetry sidescan ("SBS"). In SBS, a vertical linear array of two sets of transducers is used. Differential phase between the two sets of transducers in the array can be used to directly estimate the backscatter angle-of-arrival as a function of time instead of relying on the assumption that the seafloor is flat. Because .theta. is measured rather than assumed, SBS can, under favourable backscatter conditions, provide an accurate geometric representation of the imaging surface and can therefore be used for quantitative bathymetric mapping as well as qualitative imaging.
Both SBS and conventional sidescan sonar presume one scatterer per range cell. In practice, multiple scatterers may actually contribute to the backscatter signal measured at a particular instant. The backscattered signals from such multiple scatterers can interfere at the receive transducers. If the amplitude of the backscatter is corrupted by scatterer interference then the image is said to include "scintillation" or "speckle". Similarly, if the differential phase of the backscatter is corrupted by scatterer interference the image is said to include "glint". Both scintillation and glint are well known sources of distortion not only in underwater acoustic imaging and mapping systems but also in terrestrial and space radar imaging and mapping systems.
There is a need for acoustic imaging methods and apparatus which provide better spatial resolution than is currently available. There is also a need for acoustic imaging methods which reduce the incidence of artifacts produced by conventional sidescan systems or SBS systems.